Prefer a parameterized effect type and cats-effect type classes over specializing on a task. (In versions before cats-effect is on the classpath, specialize on use fs2.Task or scalaz.concurrent.Task.)

// Good
def apply[F[_]](service: HttpService[F])(implicit F: Monad[F]): HttpService[F]

// Bad
def apply(service: HttpService[Task]): HttpService[Task]

For examples and tutorials, use cats.effect.IO wherever a concrete effect is needed.


Prefer standard library types such as Option and List to invariant replacements from libraries such as Scalaz or Dogs.

When a list must not be empty, use cats.data.NonEmptyList.


Many parts of the HTTP spec require case-insensitive semantics. Use org.http4s.util.CaseInsensitiveString to represent these. This is important to get correct matching semantics when using case class extractors.

Case classes


The apply method of a case class companion should be total. If this is impossible for the product type, create a sealed abstract class and define alternate constructors in the companion object. Make the implementation of the sealed abstract class private.

Consider a macro for the apply method if it is partial, but literal arguments can be validated at compile time.

Safe constructors

Constructors that take an alternate type A should be named fromA. This includes constructors that return a value as a ParseResult.

case class Foo(seconds: Long)

object Foo {
  def fromFiniteDuration(d: FiniteDuration): Foo =
  def fromString(s: String): ParseResult[Foo] =
    try s.toLong
    catch { case e: NumberFormatException => 
      new ParseFailure("not a long") 

Prefer fromString to parse.

Unsafe constructors

All constructors that are partial on their input should be prefixed with unsafe.

// Good
def fromLong(l: Long): ParseResult[Foo] =
  if (l < 0) Left(ParseFailure("l must be non-negative"))
  else Right(new Foo(l))
def unsafeFromLong(l: Long): Foo = 
  fromLong(l).fold(throw _, identity)

// Bad
def fromLong(l: Long): ParseResult[Foo] =
  if (l < 0) throw new ParseFailure("crash boom bang")
  else Right(new Foo(l))

Constructors prefixed with from may return either a ParseResult[A] or, if total, A.


We use Specs2 for example-based testing and its integration with scalacheck for property testing. Property tests and arbitrary instances are encouraged.